Polyesters have widely been used as synthetic fibers, because of having many excellent characteristics. However, the polyester fibers have defects comprising inferior color development and inferior clarity, because of not having depth in color on coloration in comparison with natural fibers such as wool or silk, cellulose fibers such as rayon or acetate fibers, acrylic fibers, and the like.
In order to solve the defects, the improvements of dyes and the chemical modifications of polyesters have been tried, but a sufficient effect has not been obtained by any of the improvements and the chemical modifications.
On the other hand, as a try for imparting unevenness to the surfaces of polyester fibers to give depth to a color on coloration has been proposed, for example, a method for treating (alkali weight reduction) the fibers comprising a polyester containing polyoxyethylene glycol and/or a sulfonic acid compound with an aqueous solution of a basic compound to form wrinkle-like micropores arranged in the axial directions of the fibers.
However, a color depth-improving effect has not been recognized on the fibers obtained by this method, and the lowering in a visual concentration has contrarily been recognized.
Namely, when the alkali weight reduction is insufficient in this method, the color depth-improving effect is never recognized, because the unevenness is formed only in the surface-near places of the fibers. On the other hand, when the alkali weight reduction is sufficient, the color depth is not improved, and a visual concentration is also lowered, because light is perhaps irregularly reflected. Even when the fibers are colored in a deep color, the fibers appear whitish. In addition, the strengths of the obtained fibers are remarkably lowered, and the fibers are easily fibrillated, whereby the fibers can not resist to their employments.
Further, proposed has been a method [JP-A 54-120728 (hereinafter, JP-A means “Japanese Unexamined Patent Publication”)] for subjecting a fiber comprising a polyester containing inorganic microparticles such as silica having particle diameters of not more than 80 nm to an alkali weight reduction treatment to form irregular concaves and convexes having sizes of 0.2 to 0.7 μm and further form microconcaves and microconvexes having sizes of 0.05 to 0.2 μm in the concaves and the convexes, thus improving the depth of color. However, a color depth-improving effect is also insufficient by the method, and the fiber has additionally a defect that the fiber is easily fibrillated, because of being made in such the extremely complicated uneven form.
In order to solve such the defects, proposed has been a method for producing synthetic fibers having micropores, characterized by adding (a) a metal-containing phosphorous compound represented by the below-described formula (1) (wherein, R1 and R2 are each a monovalent organic group, and R1 and R2 may be identical or different each other; m is 1, when M is an alkali metal, or ½, when M is an alkaline earth metal) and (b) an alkaline earth metal in an amount of 0.5 to 1.2 moles per mole of said metal-containing phosphorous compound without preliminarily reacting the compound (a) with the alkaline earth metal (b), at a step before a polyester synthesis reaction is finished, finishing the polyester synthesis reaction, melt-spinning the obtained polyester, and then treating the obtained fibers with an aqueous solution of a basic compound to reduce a weight of not less than 2 percent by weight. By the method, the polyester fiber having practically excellent color depth can be obtained.

According to the method, insoluble particles can surely be produced in the polyester in a homogeneously ultra fine particle-dispersed state during the reaction. However, the method has a problem that coarse particles are liable to be produced by the slight difference of addition conditions such as a temperature on the addition of the additive and a speed on the addition of the additive. Namely, since the once produced coarse particles do not disappear, the coarse particles cause the clogging of a filter in a polymerization process, and the pressure rise of a pack and the breakage of fibers in a spinning process. Thereby, the method has a problem that it is extremely difficult to perform a continuous production by a batch type production method.
Furthermore, since the alkali metal or the alkaline earth metal is added to the reaction system in this method, the metals accelerate the hydrolysis of the polyester component. Consequently, the carboxyl terminal concentration of the polyester is enhanced, and the thermal stability of the resin composition is insufficient. Additionally, a vicious circle that the carboxyl terminal groups further promote the coagulation of the microparticles is caused.
In recent years, a direct esterification production method using a dicarboxylic acid instead of the so-called organic carboxylate as an acid component has mainly been adopted, from reasons such as cost down, the decrease of impurities, and the improvement of the hue of the polyester.
However, since carboxyl terminal groups do substantially not disappear from a raw material-charging time to a polycondensation reaction-starting time in this method, the employment of an alkali metal compound or an alkaline earth metal compound as a raw material for forming a metal-containing phosphorous compound causes the formation and precipitation of a metal carboxylate, a reaction failure, and the production of scaly coarse foreign matters in large amounts. Therefore, formation processes such as a spinning process can substantially not be carried out, and the fact is that such the composition is produced only from the so-called organic carboxylate as a raw material, thereby, enhancing the production cost.